1. Field of the Invention
The present invention relates to an apparatus for the exact position feedback of a double-acting power piston in a hydraulic power cylinder subjected to extreme environmental conditions, especially high temperatures, comprising a double-acting detecting (measuring) or follower cylinder having a volume equivalent to that of said power cylinder and including a detecting piston the position of which can be sensed by electric means, wherein a pair of working spaces of power and detecting cylinders are in fluid communication with each other via a connecting pipe, while the respective other working spaces are adapted to be connected to a (pressurized) fluid pump or a reservoir through a control valve.
2. Description of the Prior Art
In an apparatus of this type, it has been proposed to connect to the connecting pipe between the two working spaces of the power cylinder and the detecting or measuring cylinder, which are in fluid communication with each other, a pipe or line directly connected to the pump and having disposed therein a (pressure limiting) relief valve. Furthermore, according to this proposal, said connecting pipe was joined to another pipe or line leading directly to the reservoir and likewise including a relief valve for the return of excessive pressurized medium from the two interconnected working spaces. The first mentioned relief valve, controlled directly by the pump pressure, is intended to open each time when (pressurized) fluid (leakage) losses from the pair of interconnected working spaces of the power and detecting cylinders are to be compensated for. Tests have revealed that adjustment of the (pressure limiting) relief valves is extremely difficult, and that these valves greatly vary their settings in the course of operation owing to varying volume flows acting upon them. In addition, it has been found that when an excess amount of liquid is present in the fluid interconnected working spaces of power and detecting cylinders, a pressure is built up in the connecting pipe due to the direct pressurizing by the pump of the piston which has not yet reached its terminal position, which pressure is equal to the (value of) setting of the relief valve in the line extending to the reservoir, before the excess volume is relieved through said valve. This means that in such case, for example, the power cylinder is operated only by the differential pressure between the pump pressure and the set pressure of the respective relief valve.
Also, the proposed construction cannot positively ensure that, for example, the power piston will actually reach the respective terminal position. In the case of a blocking of e.g. a nozzle damper of a casting ladle, being coupled to the power piston rod, the power piston comes to stop in an intermediate position, thereby simulating a terminal position by a pressure rise. Such pressure rise would result in opening of the above-mentioned pump-controlled relief valve and, thus, movement of the sensing or detecting piston to the respective final position. The detecting piston would thereby indicate a terminal position, although the power piston has not yet reached its terminal position.